In many industries that utilize wood particles or wood fibers, logs are debarked and chipped before further processing. While efforts are made to create chips of consistent size; the nature of the chipping process; the characteristics of the wood being chipped, including the size of the log, and the physical characteristics of the fiber network; as well as environmental conditions, equipment condition, and the like result in the creation of a wide range of chip sizes during the chipping process. The largest percentage of chips will likely be of acceptable size; however, overly large chips and chips smaller than the desired optimum size also are created. During subsequent chip handling and storage, chips may be further destructured, creating fines and the like.
In the paper industry, wood chips are cooked in a digester with chemicals at high pressure and temperature to release the cellulosic fiber from the lining that holds the fibers together. If the wood chips are either undercooked or overcooked during the digestion process, the physical characteristics of the cellulosic fiber and any resulting paper product made therefrom can be adversely affected. Therefore, it is desirable that the chips being digested are all of substantially the same size. Undersized chips will be overcooked, and oversized chips will be undercooked when the cooking process is controlled for an intermediate size chip. Various screening techniques have been developed for separating oversized and undersized chips from the acceptable chips, so that the oversized can be reduced in size and the undersized chips can be separately handled. However, it is often desirable to generally understand what percentage of chips are oversized or undersized in any large volume before screening.
It is also common for wood users to purchase chips from wood suppliers. With the increased recognition of the importance of chip quality, purchase contracts for chips may specify acceptable percentages of chips in various size ranges for any chip shipment.
For these and other reasons, it is desirable to have available a chip classifier which can quickly and effectively process a random sample of chips and separate the chips by size, to determine the percentages of chips within a variety of size ranges.
In a typical chip classifier design, a number of vertically arranged trays are provided, each tray having holes in the bottom thereof of different size. The bottom tray may be solid and stationary. The trays are arranged such that trays with larger holes in the bottom thereof are positioned at the top of the stack, and each successively lower tray has progressively smaller holes in the bottom thereof. A sample of chips is placed in the uppermost tray, and the trays are shaken by some fashion to move and rearrange chips. Chips smaller than the holes of the tray in which they are in fall through the holes into the next lower tray until the chip is received by a tray having a hole size through which the chip will not pass. Depending on the information desired, a number of trays can be used to separate the chips. In this way, overlong chips, overthick chips, acceptable chips pin chips and fine chips can all be separated from each other.
While chip classifiers, as summarized above, have been useful, there are various limitations and drawbacks to their use. Conventional chip classifiers are very labor intensive and require extended periods of time to complete a chip size classification. The trays must be handled individually, and depending on the size of the classifier, the actual classification process can be very time consuming. The trays each must be removed and assembled as necessary, and the resultant chips volumes in each tray must be separately handled and weighed. Calculations are performed by hand. Additionally, the operator is limited to performing classifications possible with the hole sizes available for the trays being utilized. As the result of varying chip characteristics, ultimate use of the chips or the specifity of information desired, it may be necessary to obtain and store a wide variety of trays for performing different classifications.